Abstract: A diversion port is often constructed on the side of the main channel for drainage. The water flow can be diverted from the channel into the field for irrigation. A curved water flow is usually formed when the water flows from the channel into the diversion port. In addition, the sudden contraction of the diversion port section can cause a drastic variation in the flow state of the water in the diversion area, thus forming outstanding high-speed and low-speed zones. This uniform distribution of flow velocity can have a great impact on the diversion flow rate and sediment distribution. The water distribution of side pipes can often experience a state of free pressure, due to the variation in the flow rate and pipe diameter. This study aims to investigate the influence of free-pressure side pipes on the hydraulic characteristics of the channel diversion zone. A series of physical model experiments were conducted to determine the three-dimensional flow velocity, turbulence intensity, diversion width, and diversion ratio of the channel diversion zone under different filling degrees. The experimental results indicate that there was no variation in the distribution of three-dimensional flow velocity and turbulence intensity at the channel separation zone under different filling degrees. But the free-pressure pipe flow posed a great influence on the magnitude of the three-dimensional flow velocity and turbulence intensity. Furthermore, the longitudinal, transverse, and vertical flow velocities all shared a gradually increasing trend from the bottom of the canal to the water surface. The longitudinal average flow velocity gradually decreased from the upstream to the downstream of the water outlet. While the transverse and vertical average flow velocities first increased and then decreased. The longitudinal, transverse, and vertical turbulence intensity first increased and then decreased. The turbulence intensity was higher in the area directly at the pipeline inlet. There were relatively high horizontal and vertical flow velocities at the diversion port. Therefore, the diversion flow of the pipe increased correspondingly. However, the vertical flow velocity decreased at the diversion port. The sediment carrying capacity of the water flow was reduced near the diversion port. At the same time, the water flow was prone to form a circulation in this area, leading to settling and accumulating the sediment in the diversion area. There was an increase in the longitudinal average flow velocity and turbulence intensity, as well as the vertical average flow velocity, of each section of the channel, as the filling degree increased. While the increase was found in the transverse average flow velocity and turbulence intensity, as well as the vertical turbulence intensity. There was an increase in the diversion width of free pressure pipe flow, with the increase of filling degree. In the rectangular diversion ports, the diversion width gradually increased from the water surface downwards. By contrast, there was a more complex variation in the diversion width of free-pressure pipes along the water depth direction. The water dividing width was required to formulate the free-pressure pipe flow rather than the original formula. Once the filling degree was less than 0.5, the dividing width gradually decreased from the water surface downwards. When the filling degree was greater than 0.5, the dividing width first increased and then decreased from the water surface downwards. When the flow rate of the main channel was constant, the diversion ratio increased with the increase of filling degree; When the filling degree was constant, the diversion ratio decreased, as the flow rate of the main channel increased. Therefore, the water depth in the main channel was improved the pipeline diversion and flow at the constant inflow in the actual water supply. The finding can provide the technical support to design and maintain the canal water diversion in fields.